fp-go/v2

fp-go V2: Enhanced Functional Programming for Go 1.24+

fp-go is a comprehensive functional programming library for Go, bringing type-safe functional patterns inspired by fp-ts to the Go ecosystem. Version 2 leverages generic type aliases introduced in Go 1.24, providing a more ergonomic and streamlined API.

📚 Table of Contents

• Overview
• Features
• Requirements
• Installation
• Quick Start
• Breaking Changes
• Key Improvements
• Migration Guide
• What's New
• Documentation
• Contributing
• License

🎯 Overview

fp-go brings the power of functional programming to Go with:

• Type-safe abstractions - Monads, Functors, Applicatives, and more
• Composable operations - Build complex logic from simple, reusable functions
• Error handling - Elegant error management with Either, Result, and IOEither
• Lazy evaluation - Control when and how computations execute
• Optics - Powerful lens, prism, and traversal operations for immutable data manipulation

✨ Features

• 🔒 Type Safety - Leverage Go's generics for compile-time guarantees
• 🧩 Composability - Chain operations naturally with functional composition
• 📦 Rich Type System - Option, Either, Result, IO, Reader, and more
• 🎯 Practical - Designed for real-world Go applications
• 🚀 Performance - Zero-cost abstractions where possible
• 📖 Well-documented - Comprehensive API documentation and examples
• 🧪 Battle-tested - Extensive test coverage

🔧 Requirements

• Go 1.24 or later (for generic type alias support)

📦 Installation

go get github.com/IBM/fp-go/v2

🚀 Quick Start

Working with Option

package main

import (
    "fmt"
    "github.com/IBM/fp-go/v2/option"
)

func main() {
    // Create an Option
    some := option.Some(42)
    none := option.None[int]()
    
    // Map over values
    doubled := option.Map(N.Mul(2))(some)
    fmt.Println(option.GetOrElse(0)(doubled)) // Output: 84
    
    // Chain operations
    result := option.Chain(func(x int) option.Option[string] {
        if x > 0 {
            return option.Some(fmt.Sprintf("Positive: %d", x))
        }
        return option.None[string]()
    })(some)
    
    fmt.Println(option.GetOrElse("No value")(result)) // Output: Positive: 42
}

Error Handling with Result

package main

import (
    "errors"
    "fmt"
    "github.com/IBM/fp-go/v2/result"
)

func divide(a, b int) result.Result[int] {
    if b == 0 {
        return result.Error[int](errors.New("division by zero"))
    }
    return result.Ok(a / b)
}

func main() {
    res := divide(10, 2)
    
    // Pattern match on the result
    result.Fold(
        func(err error) { fmt.Println("Error:", err) },
        func(val int) { fmt.Println("Result:", val) },
    )(res)
    // Output: Result: 5
    
    // Or use GetOrElse for a default value
    value := result.GetOrElse(0)(divide(10, 0))
    fmt.Println("Value:", value) // Output: Value: 0
}

Composing IO Operations

package main

import (
    "fmt"
    "github.com/IBM/fp-go/v2/io"
)

func main() {
    // Define pure IO operations
    readInput := io.MakeIO(func() string {
        return "Hello, fp-go!"
    })
    
    // Transform the result
    uppercase := io.Map(func(s string) string {
        return fmt.Sprintf(">>> %s <<<", s)
    })(readInput)
    
    // Execute the IO operation
    result := uppercase()
    fmt.Println(result) // Output: >>> Hello, fp-go! <<<
}

From V1 to V2

1. Generic Type Aliases

V2 uses generic type aliases which require Go 1.24+. This is the most significant change and enables cleaner type definitions.

V1:

type ReaderIOEither[R, E, A any] RD.Reader[R, IOE.IOEither[E, A]]

V2:

type ReaderIOEither[R, E, A any] = RD.Reader[R, IOE.IOEither[E, A]]

2. Generic Type Parameter Ordering

Type parameters that cannot be inferred from function arguments now come first, improving type inference.

V1:

// Ap in V1 - less intuitive ordering
func Ap[R, E, A, B any](fa ReaderIOEither[R, E, A]) func(ReaderIOEither[R, E, func(A) B]) ReaderIOEither[R, E, B]

V2:

// Ap in V2 - B comes first as it cannot be inferred
func Ap[B, R, E, A any](fa ReaderIOEither[R, E, A]) func(ReaderIOEither[R, E, func(A) B]) ReaderIOEither[R, E, B]

This change allows the Go compiler to infer more types automatically, reducing the need for explicit type parameters.

3. Pair Monad Semantics

Monadic operations for Pair now operate on the second argument to align with the Haskell definition.

V1:

// Operations on first element
pair := MakePair(1, "hello")
result := Map(N.Mul(2))(pair) // Pair(2, "hello")

V2:

// Operations on second element (Haskell-compatible)
pair := MakePair(1, "hello")
result := Map(func(s string) string { return s + "!" })(pair) // Pair(1, "hello!")

4. Endomorphism Compose Semantics

The Compose function for endomorphisms now follows mathematical function composition (right-to-left execution), aligning with standard functional programming conventions.

V1:

// Compose executed left-to-right
double := N.Mul(2)
increment := func(x int) int { return x + 1 }
composed := Compose(double, increment)
result := composed(5) // (5 * 2) + 1 = 11

V2:

// Compose executes RIGHT-TO-LEFT (mathematical composition)
double := N.Mul(2)
increment := func(x int) int { return x + 1 }
composed := Compose(double, increment)
result := composed(5) // (5 + 1) * 2 = 12

// Use MonadChain for LEFT-TO-RIGHT execution
chained := MonadChain(double, increment)
result2 := chained(5) // (5 * 2) + 1 = 11

Key Difference:

• Compose(f, g) now means f ∘ g, which applies g first, then f (right-to-left)
• MonadChain(f, g) applies f first, then g (left-to-right)

✨ Key Improvements

1. Simplified Type Declarations

Generic type aliases eliminate the need for namespace imports in type declarations.

V1 Approach:

import (
    ET "github.com/IBM/fp-go/either"
    OPT "github.com/IBM/fp-go/option"
)

func processData(input string) ET.Either[error, OPT.Option[int]] {
    // implementation
}

V2 Approach:

import (
    "github.com/IBM/fp-go/v2/result"
    "github.com/IBM/fp-go/v2/option"
)

// Define type aliases once
type Result[A any] = result.Result[A]
type Option[A any] = option.Option[A]

// Use them throughout your codebase
func processData(input string) Result[Option[int]] {
    // implementation
}

2. No More generic Subpackages

The library implementation no longer requires separate generic subpackages, making the codebase simpler and easier to understand.

V1 Structure:

either/
  either.go
  generic/
    either.go  // Generic implementation

V2 Structure:

either/
  either.go  // Single, clean implementation

3. Better Type Inference

The reordered type parameters allow the Go compiler to infer more types automatically:

V1:

// Often need explicit type parameters
result := Map[Context, error, int, string](transform)(value)

V2:

// Compiler can infer more types
result := Map(transform)(value)  // Cleaner!

🚀 Migration Guide

Step 1: Update Go Version

Ensure you're using Go 1.24 or later:

go version  # Should show go1.24 or higher

Step 2: Update Import Paths

Change all import paths from github.com/IBM/fp-go to github.com/IBM/fp-go/v2:

Before:

import (
    "github.com/IBM/fp-go/either"
    "github.com/IBM/fp-go/option"
)

After:

import (
    "github.com/IBM/fp-go/v2/either"
    "github.com/IBM/fp-go/v2/option"
)

Step 3: Remove generic Subpackage Imports

If you were using generic subpackages, remove them:

Before:

import (
    E "github.com/IBM/fp-go/either/generic"
)

After:

import (
    "github.com/IBM/fp-go/v2/either"
)

Step 4: Update Type Parameter Order

Review functions like Ap where type parameter order has changed. The compiler will help identify these:

Before:

result := Ap[Context, error, int, string](value)(funcInContext)

After:

result := Ap[string, Context, error, int](value)(funcInContext)
// Or better yet, let the compiler infer:
result := Ap(value)(funcInContext)

Step 5: Update Pair Operations

If you're using Pair, update operations to work on the second element:

Before (V1):

pair := MakePair(42, "data")
// Map operates on first element
result := Map(N.Mul(2))(pair)

After (V2):

pair := MakePair(42, "data")
// Map operates on second element
result := Map(func(s string) string { return s + "!" })(pair)

Step 6: Simplify Type Aliases

Create project-wide type aliases for common patterns:

// types.go - Define once, use everywhere
package myapp

import (
    "github.com/IBM/fp-go/v2/result"
    "github.com/IBM/fp-go/v2/option"
    "github.com/IBM/fp-go/v2/ioresult"
)

type Result[A any] = result.Result[A]
type Option[A any] = option.Option[A]
type IOResult[A any] = ioresult.IOResult[A]

🆕 What's New

Cleaner API Surface

The elimination of generic subpackages means:

• Fewer imports to manage
• Simpler package structure
• Easier to navigate documentation
• More intuitive API

Example: Before and After

V1 Complex Example:

import (
    ET "github.com/IBM/fp-go/either"
    EG "github.com/IBM/fp-go/either/generic"
    IOET "github.com/IBM/fp-go/ioeither"
    IOEG "github.com/IBM/fp-go/ioeither/generic"
)

func process() IOET.IOEither[error, string] {
    return IOEG.Map[error, int, string](
        strconv.Itoa,
    )(fetchData())
}

V2 Simplified Example:

import (
    "strconv"
    "github.com/IBM/fp-go/v2/ioresult"
)

type IOResult[A any] = ioresult.IOResult[A]

func process() IOResult[string] {
    return ioresult.Map(
        strconv.Itoa,
    )(fetchData())
}

📚 Documentation

• API Documentation - Complete API reference
• Code Samples - Practical examples and use cases
• Go 1.24 Release Notes - Information about generic type aliases

Core Modules

• Option - Represent optional values without nil
• Either - Type-safe error handling with left/right values
• Result - Simplified Either with error as left type
• IO - Lazy evaluation and side effect management
• IOEither - Combine IO with error handling
• Reader - Dependency injection pattern
• ReaderIOEither - Combine Reader, IO, and Either for complex workflows
• Array - Functional array operations
• Record - Functional record/map operations
• Optics - Lens, Prism, Optional, and Traversal for immutable updates

🤔 Should I Migrate?

Migrate to V2 if:

• ✅ You can use Go 1.24+
• ✅ You want cleaner, more maintainable code
• ✅ You want better type inference
• ✅ You're starting a new project

Stay on V1 if:

• ⚠️ You're locked to Go < 1.24
• ⚠️ Migration effort outweighs benefits for your project
• ⚠️ You need stability in production (V2 is newer)

🤝 Contributing

Contributions are welcome! Here's how you can help:

1. Report bugs - Open an issue with a clear description and reproduction steps
2. Suggest features - Share your ideas for improvements
3. Submit PRs - Fix bugs or add features (please discuss major changes first)
4. Improve docs - Help make the documentation clearer and more comprehensive

Please read our contribution guidelines before submitting pull requests.

🐛 Issues and Feedback

Found a bug or have a suggestion? Please open an issue on GitHub.

📄 License

This project is licensed under the Apache License 2.0. See the LICENSE file for details.

---

Made with ❤️ by IBM